There are two general approaches to monitoring the pressure in vehicle tires: direct and indirect. Direct tire pressure monitoring systems (TPMSs) typically comprise a wheel module having one or more sensors and electronics mounted in or to the tire to directly measure the tire's pressure and wirelessly transmit measurement data to the vehicle.
Indirect TPMSs generally utilize information from other vehicle sensors and/or systems to indirectly estimate a tire's pressure without TPMS sensors or electronics being located in the tire. Indirect TPMS is attractive because it can be more cost-efficient than direct TPMS. One conventional indirect TPMS uses wheel speed signals from the anti-lock brake system (ABS). For a typical passenger vehicle having four tires, the indirect TPMS compares the four wheel speed signals to determine whether a wheel is rotating faster because of a loss of pressure and related decreased diameter. One drawback to some of these indirect systems is that the systems cannot detect whether all wheels have lost pressure over time because the values are compared.
Further, the quality of the signals from the ABS or other vehicle system(s) is important for indirect TPMS. Conventional indirect TPMS typically use digital signals from the ABS representing the clock generated by a rotating pole wheel. The digital signals are derived from an analog Hall signal by detecting the minimum and maximum values of the signal and determining the zero-crossing points. Unfortunately, valuable information about resonance of the tire and higher-order harmonics on the clock signal is lost by using the derived digital signals. Conventional systems therefore must attempt to recover higher-order harmonics in the ABS clock signal, though with only limited performance results.
Therefore, there is a need for improved systems and methods for indirect tire pressure monitoring.